The present invention relates to processes for improving the surfaces of devices to be surgically implanted in living bone, and to implant devices having the improved surfaces.
The success of prosthetic devices surgically implanted in living bone depends substantially entirely on achieving and maintaining an enduring bond between the confronting surfaces of the device and the host bone. Surgical procedures for preparing living bone to receive a surgically implanted prosthetic device have been known for twenty years or more, but considerable controversy remains concerning the ideal properties of the surface of the device which confronts the host bone.
It is known through clinical experience extending over several decades that titanium and its dilute alloys have the requisite biocompatability with living bone to be acceptable materials for use in making surgically implantable prosthetic devices, when the site of installation is properly prepared to receive them. There is, however, less certainty about the ideal physical properties of the surfaces of the prosthetic devices which confront the host bone. For example, the endosseous dental implant made of titanium enjoys sufficient predictable success to have become the artificial root most frequently chosen for restoring dentition to edentulous patients, but that success depends in part on the micromorphologic nature of the surface of the implant which comes in contact with the host bone. Because there is no standard for the surface micromorphology of dental implants, the surfaces of commercial implants have a wide range of available textures. It is known that osseointegration of dental implants is dependent, in part, on the attachment and spreading of osteoblast-like cells on the implant surface. It appears that such cells will attach more readily to rough surfaces than to smooth surfaces, but an optimum surface for long-term stability has not yet been defined.
Wilke, H. J. et al. have demonstrated that it is possible to influence the holding power of implants by altering surface structure morphology: xe2x80x9cThe Influence of Various Titanium Surfaces on the Interface Strength between Implants and Bonexe2x80x9d, Advances in Biomaterials, Vol. 9, pp. 309-314, Elsevier Science Publishers BV, Amsterdam, 1990. While showing that increased surface roughness appeared to provide stronger anchoring, these authors comment that it xe2x80x9ccannot be inferred exclusively from the roughness of a surface as shown in this experiment. Obviously the shear strength is also dependent on the kind of roughness and local dimensions in the rough surface which can be modified by chemical treatment.xe2x80x9d
Buser, D. et al., xe2x80x9cInfluence of Surface Characteristics on Bone Integration of Titanium Implantsxe2x80x9d, Journal of Biomedical Materials Research, Vol. 25, pp. 889-902, John Wiley and Sons, Inc., 1991, reports the examination of bone reactions to titanium implants with various surface characteristics to extend the biomechanical results reported by Wilke et al. The authors state that smooth and titanium plasma sprayed (xe2x80x9cTPSxe2x80x9d) implant surfaces were compared to implant surfaces produced by alternative techniques such as sandblasting, sandblasting combined with acid treatment, and plasma-coating with HA. The evaluation was performed with histomorphometric analyses measuring the extent of the bone-implant interface in cancellous bone. The authors state, xe2x80x9cIt can be concluded that the extent of bone-implant interface is positively correlated with an increasing roughness of the implant surface.xe2x80x9d
Prior processes that have been used in attempts to achieve biocompatible surfaces on surgically implantable prosthetic devices have taken many forms, including acid etching, ion etching, chemical milling, laser etching, and spark erosion, as well as coating, cladding and plating the surface with various materials, for example, bone-compatible apatite materials such as hydroxyapatite or whitlockite or bone-derived materials. Examples of U.S. patents in this area are U.S. Pat. No. 3,855,638 issued to Robert M. Pilliar Dec. 24, 1974 and U.S. Pat. No. 4,818,559 issued to Hama et al. Apr. 4, 1989. A process of ion-beam sputter modification of the surface of biological implants is described by Weigand, A. J. et al. in J. Vac. Soc. Technol., Vol. 14, No. 1, January/February 1977, pp. 326-331.
As Buser et al. point out (Ibid p. 890), the percentage of bone-implant contact necessary to create sufficient anchorage to permit successful implant function as a load-bearing device over time remains unclear. Likewise, Wennerberg et al., xe2x80x9cDesign and Surface Characteristics of 13 Commercially Available Oral Implant Systemsxe2x80x9d, Int. J. Oral Maxillofacial Implants 1993, 8:622-633, show that the different implants studied varied considerably in surface topography, and comment: xe2x80x9cWhich of the surface roughness parameters that will best describe and predict the outcome of an implant is not knownxe2x80x9d (p. 632).
Radio-frequency glow-discharge treatment, also referred to as plasma-cleaning (xe2x80x9cPCxe2x80x9d) treatment, is discussed in Swart, K. M. et al., xe2x80x9cShort-term Plasma-cleaning Treatments Enhance in vitro Osteoblast Attachment to Titaniumxe2x80x9d, Joural of Oral Implantology, Vol. XVIII, No. 2 (1992), pp. 130-137. These authors comment that gas plasmas may be used to strip away organic contaminants and thin existing oxides. Their conclusions suggest that short-term PC treatments may produce a relatively contaminant-free, highly wettable surface. U.S. Pat. No. 5,071,351, issued Dec. 10, 1991, and U.S. Pat. No. 5,188,800, issued Feb. 23, 1993, both owned by the assignee of the present application, describe and claim methods and means for PC cleaning of a surgical implant to provide a contact angle of less than 20 degrees.
Copending application Ser. No. 08/149,905, filed Nov. 10, 1993, owned by the assignee of the present application, describes and claims inventions for improving the surfaces of surgically implantable devices which employ, among other features, impacting the surface with particles of the same material as the device to form the surface into a desired pattern of roughness.
It is a primary object of the present invention to produce an implant surface having a roughness that is substantially uniform over the area of the implant that is intended to bond to the bone in which the implant is placed.
It is a further object of this invention to provide an improved surgically implantable device having on its surface a substantially uniform micromorphology.
It is another object of the invention to provide a process or processes for manufacturing such improved implant devices.
It is an additional object of the invention to provide such improved implant devices which can be manufactured without contaminating the surfaces thereof.
It is a more specific object of the invention to provide an improved etch-solution process that will result in a substantially uniform surface topography on surgically implantable devices.
In accordance with the present invention, the foregoing objectives are realized by removing the native oxide layer from the surface of a titanium implant to provide a surface that can be further treated to produce a substantially uniform surface texture or roughness, and then performing a further, and different, treatment of the resulting surface substantially in the absence of unreacted oxygen. The removal of the native oxide layer may be effected by any desired technique, but is preferably effected by immersing the implant in hydrofluoric acid under conditions which remove the native oxide quickly while maintaining a substantially uniform surface on the implant. The further treatment is different from the treatment used to remove the native oxide layer and produces a desired uniform surface texture, preferably acid etching the surface remaining after removal of the native oxide layer. To enhance the bonding of the implant to the bone in which it is implanted, a bone-growth-enhancing material, such as bone minerals, hydroxyapatite, whitlockite, or bone morphogenic proteins, may be deposited on the treated surface. The implant is preferably maintained in an oxygen-free environment following removal of the native oxide layer, in order to minimize the opportunity for oxide to re-form before the subsequent treatment is performed.